There's a discussion about this article on Hacker News: https://news.ycombinator.com/item?id=26724522 that's worth looking at.

This Medium post is obscurely written. The author is apparently claiming to have witnessed someone he knew have sex with an underage girl. But this is from the alleged witness, not the girl. It's hard to tell, but apparently on this basis he claims that the person who was associated with Facebook is a rapist. Whether this was consensual sex with an underage girl or assault is not clear.

Bottom line, take all of this with a grain of salt.